This invention relates to a process of producing phosphate coatings on a metal surface of iron or steel by treating the surface with an aqueous phosphating solution on the basis of magnesium phosphate which contains an accelerator.
In the metal-working industry, phosphate coatings are produced on a large scale by a treatment with aqueous phosphating solutions. The phosphate layers which are formed on the so treated metal surfaces serve particularly for protection against corrosion, as a base for paints, to facilitate cold-working operations and also to reduce friction.
The phosphating solutions employed in such processes may have, e.g., a pH between about 1.8 and 3.8 and may contain zinc ions and phosphate ions as process-determining components. In addition to zinc, other cations, such as ammonium, calcium, cobalt, iron, potassium, copper, sodium, magnesium, and manganese, may be present. In order to accelerate the formation of the phosphate layers, the phosphating baths usually contain oxidizers, such as bromate, chlorate, nitrate, nitrite, organic nitro compounds, perborate, persulfate and hydrogen peroxide. Additionally, an oxygen-containing gas may be used to oxidize iron (II) to iron (III).
A phosphating process of a different kind is the so-called iron-phosphating process or "non-layer-forming phosphating process", in which phosphating solutions usually containing alkali phosphate and, optionally containing phosphates of magnesium, calcium and barium, are employed. Such processes can be carried out at low cost in a comparatively simple manner. The resulting phosphate coating quality is sufficient in many cases, particularly on articles which will not be subjected to strong corrosive action.
The two kinds of processes which have been outlined hereinbefore have in common that the phosphating solutions are contacted with the workpiece surface to be treated by dipping, flooding or spraying. Phosphate layers which are firmly intergrown with the metal are formed during the contacting time by a chemical reaction. The contact period may be from a few seconds to about 15 minutes. The phosphating is followed by a thorough rinsing because any residual phosphating solution left on the treated surface is usually disturbing.
In the so-called "non-layer-forming phosphating process" it is known that the resistance of metal surfaces to corrosive action can be improved by the application of a phosphate coating by a treatment with solutions which mainly contain phosphoric acid and/or a non-layer-forming phosphate (British Patent Specification 517,049). Alkali phosphate, ammonium phosphate, and magnesium phosphate have been mentioned as non-layer-forming phosphates because they are conveniently available and tend to form lesser amounts of sludge than the phosphates of zinc, manganese, cadmium, and iron. The solutions additionally may contain small amounts of nitrite, nitrate or sulfite and are permitted to contain only small amounts of layer-forming phosphates.
However, that process has the disadvantage that the abovementioned contents of nitrite, nitrate, and sulfite and the optional presence of alkali ion or ammonium ion as a non-layer-forming cation can result in the formation of waste waters from the subsequent rinsing. Those waste waters require an expensive aftertreatment before they can be disposed of. The expensive aftertreatment offsets a considerable part of the advantages afforded by the inherently inexpensive "non-layer-forming phosphating processes".
It is an object of the invention to provide a non-layer-forming phosphating process which is free of the known disadvantages, particularly those outlined hereinbefore, but which is inexpensive and can be conveniently carried out and monitored.